A light waveguide in a single crystal of LiNbO.sub.3 doped with Ti is prepared by placing a thin layer of Ti on the surface of the LiNbO.sub.3 and heating to cause the Ti to diffuse into the LiNbO.sub.3 substrate. A Ti-rich channel penetrating several microns below the LiNbO.sub.3 surface results which provides a light waveguide.
Although many different optical operations with integrated optics can be performed in a single LiNbO.sub.3 substrate, such as optical modulation, switching, polarization rotation, wavelength multiplexing, and the like, the optical operation of amplification has heretofore not been achieved in such an integrated substrate.
Optical gain (or amplification) has been demonstrated using Nd-doped LiNbO.sub.3 grown from a melt (Nd:LiNbO.sub.3). Also, efforts are underway to grow Er:LiNbO.sub.3 for optical amplification at a wavelength of about 1500 nm (a useful wavelength for fiber optic telecommunications). However, such methods and devices are not suitable for integrated optics because of the inherent absorption losses that occur in a uniformly doped substrate.
A technique is needed by which a single crystal of LiNbO.sub.3 can be treated to produce therein a spatially localized absorbing integrated optical device, particularly a light amplifier.